Some electronic devices comprise microphone components for capturing audio. A microphone component of an electronic device is typically integral with the electronic device and is located within the electronic device such that audio from the surrounding environment of the electronic device is captured.
The microphone component of the electronic device may comprise a membrane which moves in response to sound incident thereon. The movement of the membrane is detected and circuitry of the microphone component may generate an audio signal.
When capturing audio from the environment of the electronic device the membrane of the microphone component may be subject to other vibrations of the electronic device. For example, structural born mechanical vibrations of the electronic device can cause movement of the membrane. The movement of the membrane due to mechanical vibrations may be converted into the audio signal. This means that mechanical vibrations such as handing of the electronic device, movement of other components within the electronic device or other external mechanical vibrations of the electronic device are represented as noise in the audio signal. The noise in an audio signal not due to sound can therefore significantly deteriorate the audio signal which may result in a bad user experience.
It is known to isolate a microphone from mechanical vibrations of an electronic device using vibration dampening material such as rubber gaskets immediately around the microphone component. However some electronic devices are small in size and the amount of space available within the electronic device to fit vibration dampening material is limited. This means effectively isolating mechanical vibration from small and lightweight microphone components in small electronic devices can be difficult to achieve.
Another known mechanical arrangement mounts a microphone component on a floating back plate. The back plate is designed to vibrate together with the microphone component when the electronic device experiences mechanical vibrations. However, the differing masses of the back plate and membrane of the microphone component can cause a mismatch in the frequency response of the back plate and the frequency response of the membrane. A frequency response mismatch can lead to poor noise cancelling performance. Additionally the performance of the microphone component in an environment where the electronic device is not subject to mechanical vibrations may be degraded due to the floating back plate.
An alternative known arrangement detects the movement of an electronic device using acceleration sensors. The acceleration of the electronic device is detected and matched with an audio signal generated by the microphone component to determine which “noises” in the audio signal are due to mechanical vibrations. Digital signal processing is then applied to the audio signal in order to remove audio signals generated when the electronic device is subject to mechanical vibrations. However, the acceleration sensors can have different vibration sensitivities from the microphone membrane component at various frequencies of mechanical vibration, which can lead to poor noise cancelling performance. Furthermore production of a microphone component comprising both a membrane and an accelerometer can require non-optimal manufacturing solutions which may be costly.
Noise cancelling microphones can be used where clear communication in noisy ambient environments is required. Noise cancelling microphone designs may be a passive noise cancelling microphone or an active noise cancelling microphone.
An active noise-cancelling microphone may comprise two individual microphone elements and a circuit element for electronically differentiating two signals from the two microphone elements. The two microphone elements are arranged such that a first microphone element receives the desired speech input and the background noise present in the vicinity of the speech, and a second microphone element senses substantially only the background noise. Therefore, a noise reduced speech signal can be generated when subtracting the second microphone signal from the first microphone signal by the circuit element of the active noise-cancelling microphone.
The active noise-cancelling microphone system may use a built-in calibration function to calibrate the two microphones based on relative signal levels from the microphones. During the operation of the noise-cancelling microphone system output values of the microphones are monitored. The active noise-cancelling algorithm determines that any difference in signal level of the two microphones is due to acoustical pressure wave level differences. However, if there is a change in one microphone output caused by temperature change, and the calibration function does not compensate, then the noise cancelling algorithm would not be performing as well as expected. In fact, any condition that changes the sensitivities of the two microphones differently relative to the calibrated value will deteriorate the performance of the entire system. The sensitivity difference of the microphones in relation to each other can be caused by a relatively fast temperature difference between the microphones. This can be caused, for example, by a power amplifier in the device that heats the other microphone to e.g. 50 degrees centigrade. If the microphones are not identical they will react differently to changes in ambient temperature and this causes the sensitivity change in one more than in the other.
An alternative known arrangement is shown in FIG. 4. The arrangement involves a direct digital microphone that is constructed of a plurality of first membranes 420 each formed by a micro-machined mesh supported by a substrate 470. A second membrane 410 and a plurality of first membranes 420 are located in two different positions. A direct digital microphone that is constructed of the plurality of first membranes 420 is comprised of individual first membranes 460. The second membrane 410 is supported by a substrate 470 and positioned above the plurality of first membranes 420 to form a chamber 430 between the plurality of first membranes 420 and the second membrane 410. A pressure sensor 440 is responsive to pressure in the chamber 430. Drive electronics 450 are responsive to the pressure sensor 440 and control the positions of the plurality of first membranes 420. Polling electronics 450 are responsive to the positions of the plurality of first membranes 420 and produce a digital output signal.
Another known arrangement is shown in FIG. 5. The arrangement comprises at least two membranes with one membrane being desensitized as compared to the other membrane. Neither of these membranes are stacked, and the arrangement allows for the recording of audio at high SPL levels without saturation. There is a higher noise floor of the desensitized membrane and a smaller SNR.
The arrangement of FIG. 5 allows for operation of a mobile device during noisy conditions such as those due to wind, traffic, a crowd, etc. A high-pass electrical filter can be implemented between a microphone capsule and an ASIC in order to allow for operations in windy conditions. This, however, is an imperfect solution for at least three reasons: 1) the microphone output signal is often already saturated by wind noise, 2) the demands of preferred audio quality in non-windy environment require the high-pass filter to be set at a point which will still pass a large proportion of the wind noise, and 3) this strategy is not possible with digital microphones. Attempts have been made to use DSP circuitry to clean a windy signal from a multiple array of microphones but they have had limited effectiveness. Each membrane has a different sensitivity and each outputs a separate signal. In this example, only the signal from the less sensitive membrane has an acceptable distortion level, only that signal is selected for further processing and the other signal, which may be overly distorted due to signal clipping as the high-amplitude sound field exceeds the full scale output of the membrane and ADCs, is disregarded/dumped. Additionally, there may also be a high pass filter on one or both signal paths which can be selectively activated based on wind noise levels. The filter on the signal path that is continued may be activated to further reduce wind noise in some instances where the signal is additionally distorted in this way.
Embodiments of the application aim to address one or several of the above issues.